This invention relates to a tensioning device for tensioning the carrier of a vertical conveyor, particularly a bucket conveyor. The carrier, on which the buckets are usually equidistantly mounted, is an endless chain, or cable or strap structure trained, at opposite longitudinal ends of the conveyor, about two end drums. One of the end drums is a driving drum rotated by a power drive, while the other end drum is a tensioning drum which is shiftable towards and away from the driving drum for appropriately tensioning the carrier. The force generating component of the tensioning device may comprise a tensioning weight which exerts a force on the tensioning drum in a direction oriented away from the driving drum and which cooperates with two tensioning levers. Generally, a resilient tensioning force is transmitted to the shaft of the tensioning drum by means of tensioning spindles arranged on either side of the tensioning drum. In case the force-generating device is a weight, the latter may be arranged between the two spaced, upright ducts in which the carrier circulates with the buckets. The tensioning weight, with the intermediary of a connecting element (such as the tensioning spindles acting as pressure bars) arranged between the tensioning weight and the tensioning drum, tensions the carrier by shifting the tensioning drum into its operating position parallel to the length of the bucket conveyor in a direction away from the driving drum.
The above-outlined known tensioning devices have the disadvantage that the tensioning drum cannot be shifted parallel to its initial position during the tensioning step. If the tensioning drum shaft assumes an oblique position relative to its aligned, initial position prior to applying the tensioning force, the carrier will not run in a satisfactory manner; in case of such an oblique positioning of the tensioning drum axis, the carrier--particularly if it is a strap--has the tendency of running off the tensioning drum. It is a further disadvantage of the above-outlined prior art arrangements that in case of an elastic tensioning of the carrier, an automatic follow-up movement cannot be effected if an elastic or, in particular, a permanent elongation of the carrier occurs. As a consequence, the carrier, after a certain service period, becomes slack if the carrier tension is not continuously and periodically verified and adjusted and thus a slippage between the carrier and the driving drum may occur which leads to a significant wear and also results in an unsatisfactory guidance of the carrier.
Further, a spring bias achieved with the above-outlined prior art tensioning devices is not capable of equalizing length differences of the carrier; such length differences may occur, for example, because of non-uniform elongations of the carrier along its width and/or non-uniform elongations of the upright ducts.
While the above-outlined disadvantages can be eliminated in a large measure in a tensioning device which is equipped with a tensioning weight, during tensioning such weight-type tensioning devices nevertheless often lead to significantly greater misalignments and oblique positioning of the tensioning drum than elastic (spring operated) tensioning devices. In order to eliminate such an unsatisfactory operation, there has been developed a tensioning device which has two tensioning levers arranged essentially perpendicularly to the length dimension of the bucket carrier. One end of each tensioning lever engages the shaft of the tensioning drum, while the respective other ends are articulated to a shaft supported by appropriate bearings in the conveyor housing. Such a known tensioning device has a number of significant drawbacks.
It is first noted that the last-mentioned tensioning device is not adapted for installation between the two upright ducts; it must be arranged at the foot structure of the bucket conveyor system. Such a foot structure is substantially wider than the available space between the upright ducts. This circumstance has, among others, the disadvantage that it is practically not feasible to equip an already-existing bucket conveyor with such a known tensioning device.
It is a further disadvantage of the last-mentioned known tensioning device that during the tensioning step the tensioning drum must move along a circular arc whose radius corresponds essentially to the length of the tensioning lever, that is, to the distance between the tensioning drum axis and the shaft which supports the tensioning lever. Such a motion of the tensioning drum away from the line of alignment with regard to the axis of the driving drum is very disadvantageous. The shorter the length of the tensioning lever, the greater the deviation from the aligned position.
It is a further disadvantage of the last-named tensioning device that it permits only a limited readjustment and further, in case of relatively large elongations of the carrier (which occur particularly in case of bucket conveyors of great length and, dependent upon the carrier material may be particularly significant, for example, in case of certain straps), the known tensioning device is no longer in a position to take up in an operationally satisfactory manner the elastic and permanent elongations or expansions of the bucket carrier.